One-part moisture-curable polyurethane compositions have been used as adhesives, coatings, and sealants (see, e.g., U.S. Pat. Nos. 3,380,950, 3,380,967, 3,707,521, 3,779,794) and provide convenient application and good ultimate physical properties. For example, most U.S. automobile manufacturers utilize one-part moisture-curable polyurethane sealants to bond front and rear windshield glass to automotive passenger car bodies. The resultant bonded windshield assemblies become an integral structural part of the car body and contribute to the roof crush resistance thereof, thereby assisting the car manufacturer in meeting the requirements of DOT specification No. 216 (see 49 CFR 571.216).
For any adhesive, coating, or sealant composition, the ultimate physical properties available after cure (e.g., tensile strength, shear strength, weathering resistance, flexibility, etc.) are of great importance to the user. However, ultimate physical properties are not the only parameters by which such compositions can be evaluated. Most adhesive, coating, and sealant compositions provide a gradual buildup of physical properties during cure. It is highly desirable for such compositions to provide not only a high level of ultimate physical properties but also rapid attainment thereof. For example, a windshield sealant for use on cars manufactured by General Motors Corp. not only must pass certain physical property tests specified by the automaker but must pass such tests within 6 hours after sealant application. Similarly, adhesives, coatings, and sealants used in structural applications (e.g., building construction, general manufacturing, and the like) not only must provide good physical properties (e.g., high bond strength) but also should provide such properties as rapidly as possible in order to speed assembly times and reduce fixturing costs. In general, for an adhesive, coating, or sealant composition with a given level of physical properties, the faster the rate of property buildup, the better.
An additional important parameter by which such compositions are evaluated is the "tack-free time" thereof, that is, the time required for a sample of the composition to become non-tacky to the touch after exposure of the composition to ambient air. Compositions having rapid tack-free time enable parts bonded therewith to be subjected to subsequent operations (e.g., grinding, drilling, handling, packaging, and the like) which could contaminate parts joined with a tacky material. In general, for an adhesive, coating, or sealant having a given level of physical properties, the faster the tack-free time, the better.
A further important parameter by which such compositions are evaluated is the shelf life thereof, that is, the amount of time the compositions can be stored under typical storage conditions without significant loss of handling properties when uncured, physical properties when cured, or cure characteristics during cure. Unfortunately, compositions having rapid physical property buildup or rapid tack-free times typically also have attenuated shelf life, thus making it very difficult to obtain rapid physical property buildup, rapid tack-free time, and long shelf life in a single composition.